This invention relates to digital data transmission systems and more particularly, to a phase filter for reducing the effects of the noise components altering the signals transmitted through a system in which the phase of the transmitted signals is modulated by discrete values, at discrete times.
Phase modulation is a technique widely used in data transmission systems and a detailed description thereof is provided, for example, in "Data Transmission" by W. R. Bennett and J. R. Davey, chapter 10, McGraw-Hill, New York 1965 and "Principles of Data Communications" by R. W. Lucky, J. Salz and E. J. Weldon Jr., chapter 9, McGraw-Hill, New York 1968. In those digital data transmission systems that utilize phase modulation, the digital data to be transmitted modulate the phase of a carrier at particular instants or sampling times. The direct modulation method, called "Coherent Phase Modulation", consists in making a predetermined phase absolute value to correspond to a data group or character. For instance, in an eight-phase system, i.e., in a system in which the phase of the transmitted signal can assume eight distinct discrete values, it is possible to make the eight phase absolute values .pi./8, 3.pi./8, . . . , 15.pi./8, respectively, to correspond to the eight characters 000, 001, 011, 010, 110, 111, 101 and 100 as shown in the diagram of FIG. 1a. The modulation method the most currently used, called "Differential Phase Modulation", consists in making a phase change rather than a phase absolute value to correspond to a character. Always in an eight-phase system, it is possible to make phase variations .DELTA..phi. = .pi./8, 3.pi./8, . . . , 15.pi./8 to correspond to the eight characters 000, 001, 011, . . . , 100. This type of modulation can also be illustrated by the diagram of FIG. 1a by taking the phase value of the signal emitted at the preceding sampling time as reference axis OX. The so-modulated carrier frequency is sent, through a transmission medium, to a receiver coupled thereto. At the receiver, the phase value of the signal received at sampling times is detected, and then the value of the transmitted data is extracted therefrom. This extraction is generally done by comparing the phase of the signal received at a given sampling time with a reference phase available in the receiver or with the phase of the signal received at the preceding sampling time according to the use of a coherent or differential detection, as described in the above-referenced books.
It is to be noted that phase modulation as briefly described above, is not the only one process for transmitting digital data, in which the phase of the transmitted signals represents the data. For instance, it is the case of the quadrature amplitude modulation a description of which can be found in the above-referenced book by R. W. Lucky et al, chapter 7 and more particularly in paragraph 7.1.5. Briefly, the quadrature amplitude modulation consists in modulating the amplitude of two carriers in quadrature by discrete values, said carriers being emitted in the same time. The following table shows the correspondence between the digital characters, the amplitude of each of the carriers in quadrature A and B and the phase and the amplitude of the signal resulting from the combination of these carriers, in an eight-state system illustrated by the diagram of FIG. 1b.
______________________________________ Digital Amplitude Amplitude Phase of Amplitude characters of A of B the result- of the ing signal resulting signal ______________________________________ 000 + 3 0 0 + 3 001 + 1 + 1 .pi./4 +.sqroot.2 011 0 + 3 2.pi./4 + 3 010 - 1 + 1 3.pi./4 +.sqroot.2 110 - 3 0 4.pi./4 + 3 111 - 1 - 1 5.pi./4 +.sqroot.2 101 0 - 3 6.pi./4 + 3 100 + 1 - 1 7.pi./4 +.sqroot.2 ______________________________________
From this table, it appears that, in this example, the data can be directly derived from the value of the phase of the emitted resulting signal.
It would be desirable if the emitted signals should be received without distortion in the receiver. In practice, however, the transmission media introduce disturbances such as intersymbol interference and the noise components mainly due to frequency shift, phase intercept, phase jitter and white noise, which disturbances alter the emitted signals when transmitted through the transmission medium.
Intersymbol interference is due to an interaction between successive emitted signals, which interaction is caused by amplitude and phase distortions introduced by the transmission medium. When intersymbol interference appreciably affects the quality of the received signals, it is cancelled or reduced by an appropriate device called an "Equalizer". Within the scope of this invention, it is assumed that intersymbol interference is cancelled by an appropriate equalizer, as necessary.
Frequency shift is a disturbance affecting the emitted signals when they are transmitted through a transmission medium in which they are submitted to an intermediate processing and more particularly, when telephone lines are used as the transmission medium. Said intermediate processing mainly includes the transposition of the emitted signals from one frequency band to another as required by the public network. A frequency shift f.sub.s introduces a phase shift .phi..sub.s = 2.pi.f.sub.s t, where t is the time, which phase shift directly affects the phase of the received signal.
Phase intercept is due to the presence of a difference between the actual phase of a frequency and the phase corresponding to the ideal channel phase/frequency characteristic, at the ends of the frequency bandwidth of the transmission channel. This phase intercept introduces an arbitrary constant in the received phase value.
Phase jitter results from a random noise frequency modulation of the signals when passing through the transmission medium. Often, it is due to the variation of the power sources of the devices used to carry out the above-indicated intermediate processing.
White noise is due to the additive noise in the transmission medium and to the residual intersymbol interference. It is characterized by a flat frequency spectrum with equal contributions for all the frequencies, but in which the various frequencies exhibit random phases.
These noise components have practically no effect in the low speed digital data transmission systems but avoid correct data detection in high speed systems. In those systems using phase modulation, an increase of the transmission speed is generally obtained by increasing the number of the distinct discrete values which can be assumed by the phase of the emitted signal, which increase appears as a decrease of the gap between two adjoining phase values. For instance, in a four-phase system, this gap is of 90.degree., but it is only of 22.5.degree. in a sixteen-phase system. Then it is often not possible to discriminate between two possible phase values in presence of the various above-indicated disturbances and it becomes imperative to provide a device to cancel or reduce the effects of these noise components before detecting data.
U.S. Pat. No. 3,855,539, filed Mar. 16, 1973 on behalf of Alain Croisier, which patent is assigned to the assignee of the present invention, describes a method and a device for reducing the effects of the noise components altering the phase of discrete phase modulated signals. According to this method, a correction value is subtracted from the received signal phase value. The result of this subtraction is multiplied by a first factor proportional to the number of distinct discrete values that the phase of the emitted signal can assume. The integral part of this product represents the data while the fractional part is used to provide said correction value. Said correction value is obtained by multiplying said fractional part by a second factor and by integrating the result of this last multiplication. Controlling the value of the second factor enables to minimize the effects of the various above-indicated components selectively. However, this method shows a drawback consisting in the fact that the reduction of the effect of the phase jitter is necessarily accompanied by a decrease of the performance with respect to the white noise. The curves shown in FIG. 3 of the above-indicated patent application enables one to appreciate the difficulty encountered to find a good decrease of phase jitter/increase of white noise compromise.
One of the objects of this invention is to overcome this drawback by providing an optimum phase filter for minimizing, the effects of phase intercept, frequency shift, phase jitter and white noise altering discrete phase modulated signals.
Another object of this invention is to provide an adaptive phase filter allowing to reduce the phase jitter effect to a minimum, whatever the noise modulation frequency causing said jitter, may be.
The objects of this invention are generally obtained by providing a phase filter including two decision filters which can be linked through a cascade or a parallel connection. The first decision filter cancels the phase intercept component and the phase shift component introduced by the frequency shift, and the second decision filter minimizes the random component representing phase jitter and white noise. In the first decision filter, a first error signal corresponding to an estimated value of the phase intercept and phase shift components is subtracted from the phase value of the received signal. The result of this first subtraction is applied to a detector-separator which separates the data and the residual noise component. The residual noise component is applied to a linear filter generating from the previous residual noise components, the estimated value of the phase intercept and phase shift components. In the second decision filter, a second error signal corresponding to an estimated value of the random component is subtracted from the result of the first subtraction. The result of this second subtraction is applied to a detector-separator which fetches the data and the residual random component out. Said component is applied to a linear filter which generates from the previous residual random components, the estimated value of the random component. When the phase jitter characteristics are unknown or if they are varying in time, the estimated value of the random component can be obtained by an adaptive predictive filter.
These and other objects, advantages and features of the present invention will become more readily apparent from the following specification when taken in conjunction with the drawings.